This invention relates generally to a non-impact printing apparatus and method and more particularly to an apparatus and method in which the printing fluid is ejected by volume displacement from a nozzle.
Historically printing has been done by applying ink to a specially configured key or carrier and mechanically impacting the key or carrier on a recording medium such as paper to form an impression of the carrier. More recently, non-impact printing devices have been developed, where intelligence patterns (alphanumeric characters, graphical displays, etc.) are deposited on a recording medium. Non-impact printing devices utilize a variety of methods of forming the intelligence patterns, including chemically active and chemically inert processes, using either fluids or solids as the marking or printing medium, and requiring either specially treated recording media or untreated recording media.
It has been known to print by depositing discrete droplets of printing fluid on a recording medium in a predetermined pattern. Previous attempts to achieve such a method of printing utilize a continuous stream of fluid which separates into droplets which are charged and electrostatically deflected so that they form the desired pattern on the recording medium. Such methods produce acceptable resolution only when the charge per unit mass is accurately controlled for each droplet. Two basic variations have been developed: the droplets are either given equal charge per unit mass and then deflected by an electrostatic field whose intensity is controlled by the input signal, or the droplets are given a charge per unit mass according to the input signal and then deflected using a constant electrostatic field. Existing embodiments of both of these methods require that the fluid droplets be uniform, which has proven difficult to achieve. Once the stream of uniform droplets has been attained, it is usually necessary to provide voltages in the range of 2,000 to 10,000 volts for the electrostatic field. Such voltages are difficult and expensive to produce and control. Further, the process of charging the droplets themselves sometimes causes electrolysis of the printing fluid, producing corrosive byproducts which may cause electrode deterioration.
In an effort to obtain droplets of uniform size, two basic methods have been developed in the prior art. First, the printing fluid is delivered to a nozzle at sufficient pressure to assure that a continuous jet of fluid issues from the nozzle. The jet stream is separated into droplets by using radial oscillations or vibrations induced in the nozzle itself by means of magnetostrictive drivers or piezoelectric crystals. The vibrations cause regularly spaced varicosities in the ink stream, aiding the natural tendency of the stream to separate into droplets, and making the ensuing droplets more uniform than would otherwise occur.
In another embodiment of the droplet formation process, the printing fluid is delivered to the nozzle under sufficient pressure to form a meniscus at the nozzle but not high enough to produce flow through the nozzle. In this method, the fluid is drawn from the nozzle electrostatically in a ray-like jet which is then deflected electrostatically as desired. The electrostatic field which draws the jet of fluid from the nozzle is constant, producing a continual stream of printing fluid. The stream breaks into a succession of droplets with essentially uniform mass and charge. A time-varying electrostatic field controlled by an input signal is then used to deflect the droplets as required for the formation of alphanumeric characters. All of the foregoing printing processes and mechanisms make use of a continuous flow of printing fluid, with the flow to be diverted to a reject basin or collector whenever no characters or patterns are to be printed. This results in a more complicated system for handling the flow of printing fluid than would otherwise be required.
In another type of device, piezoelectric transducers are employed to create shock waves in a horn-shaped nozzle. As the waves travel down along the nozzle from the large end to the small end, their intensity increases. The high intensity shock waves serve to eject fluid from the end of the nozzle towards and onto an associated printing medium.